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The concentration and distribution of aromatic biomarkers in coals and shales from five boreholes penetrating the Maastrichtian Mamu Formation of the Anambra Basin, southeastern Nigeria, were investigated by gas chromatography-mass spectrometryto assess the thermal maturity and organic matter input. The study focused on the variations of the relative abundances of naphthalenes, phenanthrenes, and monaromatic and triaromatic steroids identified on the mass fragmentograms. Trimethylnaphthalene(TMN) is the most abundant member of the naphthalene family while methylphenanthrene (MP) is the most abundant phenanthrene family member. The total of phenanthrenes and their isomers was greater than that of naphthalenes. The distribution of these aromatic hydrocarbons and their akyl derivatives was strongly controlled by a selective expulsion mechanism and thermal maturation of organic matter. The low dibenzothiophene/phenanthrene (DBT/PHEN) ratios (0.01-0.06), as well as the enhanced concentrations of 1,2,5-TMN relative to 1,2,7- TMN,indicates organic matter derived mainly from higher plants,and the extract ternary plot of C 27 , C 28 and C 29 monoaromatic steroids suggests a Type III and mixed Type II/III kerogen. The calculated mean vitrinite reflectance (%R m ), determined from the distributions of the isomers of methyldibenzothiophene ratio (MDR) in the rock extracts, ranged from 0.51 to 1.43. These maturity values indicate that the coal and shale extracts are marginally mature for hydrocarbon generation. Resumen La concentracion y distribucion de biomarcadores aromaticos en carbones y esquistos de cinco perforaciones en la formacion Maastrichtian Mamu de la cuenca de Anambra, en el sureste de Nigeria, fueron analizados a traves de un estudio de espectometria cromatografico y de masa del gas para medir la madurez termal y la entrada de material organico. El estudio esta enfocado en las variaciones de la abundancia relativa de naftalinas y fenantrenos, y en los esteroides monoaromaticos y triaromaticos identificados en los fragmentogramas de masas. La trimetinaftalina (TMN) es la mas abundante de la familia de las naftalinas mientras el metilfenantreno (MP) es el mas abundante de los fenantrenos. El tota de los fenantrenos y sus isomeros fue mayor que el de las naftalinas. La distribucion de estos hidrocarbones aromaticos y sus alquilos derivados fue controlada ampliamente por un mecanismo de expulsion selectiva y de la maduracion termica de material organico. La baja proporcion dibenziotofeno/fenantreno (DBT/ PHEN) (0.01-0.06), al igual que las concentraciones mejoradas de 1,2,5-TMN relativas de 1,2,7-TMN indican que la materia organica se deriva principalmente de plantas mayores, y del diagrama terniario de los esteroides monoaromaticos C 27 , C 28 y C 29 sugiere un tipo III mezclado con tipos II/III de querogenos. El valor calculado de la reflectancia de vitrinita (%Rm) determinado de la proporcion de isomeros de metildibenziotofeno (MDR) en los extractos rocosos oscila de 0.51 a 1.43. Estos valores de madurez indican que los extractos de carbones y esquistos son marginalmente maduros para la generacion de hidrocarbono.

earth sciences research journal/earth sciences research journal

Using aromatic biological markers as a tool for assessing thermal maturity of source rocks in the Campano-Maastrichtian Mamu Formation, southeastern Nigeria